Asymmetric polymer membranes can be used for gas separation, for example for the production of oxygen-enriched air, production of nitrogen-enriched streams for blanketing fuels and petrochemicals, separation of carbon dioxide or hydrogen sulfide from methane in natural gas, hydrogen recovery from ammonia plant purge streams, and removal of organic vapor from air or nitrogen. Semi-permeable asymmetric “skinned” gas separation membranes can be formed by phase inversion and solvent exchange methods. Such membranes are characterized as “asymmetric” because they comprise a thin, dense, selectively semi-permeable surface “skin” and a less dense void-containing, non-selective support region. In the support region, pore sizes range from large in the support region to very small proximate to the non-porous and selectively semi-permeable “skin.”
A commercially viable gas separation membrane combines high selectivity for the desired gas, high permeation flux, or throughput, and a long service life. Permeation flux is a measure of volumetric gas flow through a membrane. The higher the permeation flux, the smaller the membrane area required to treat a given volume of gas. Separation factor is a measure of membrane selectivity for the gas pair to be separated. It is the ratio of the fluxes of the individual gases across the membrane. For example, for oxygen/nitrogen separations, the separation factor is the ratio of oxygen flux to nitrogen flux. Since selectivity can be inversely proportional to flux, it is desirable to increase the selectivity without adversely affecting flux. Selectivity is proportional to skin thickness, but flux is inversely proportional to skin thickness. Therefore, it is desirable to increase selectivity without increasing skin thickness. It is also desirable to have gas separation membranes with long service lives under harsh conditions, for example high temperatures and exposure to corrosive gases, so that replacement costs are minimized. A large number of materials have been investigated for use in gas separation membranes. It is desirable to develop new materials that will improve selectivity without increasing skin thickness and permeation flux. It is also desirable to develop gas separation membranes with long service lives under harsh conditions.
Gas separation membranes can be formed by phase separation of solutions of polymer in solvent mixtures. The thickness, permeability, and permeation flux of the skin can be in part controlled by the choice of cosolvent. Toward this end, the solvent mixture can include a polar solvent comprising 2 to 8 carbon atoms. It is desirable to use a co-solvent other than 2-ethylhexanol. The cosolvent should be easier to remove by evaporation, and should provide gas separation membranes with advantageous skin properties of high selectivity and high flux.